This invention relates to spectrophotometers in general and more particularly to a spectrophotometer with improved curvature correction. While the invention is described herein as embodied is a dual channel atomic absorption spectrophotometer, it will be understood that it may also be applied to various other analytical instruments of both the absorption and emission analysis type.
Although the principle of atomic absorption has been known to astronomers for over one hundred and fifty years, its application to chemical analysis only began in 1955. In atomic absorption analysis, the sample is heated to a high temperature, e.g., by burning it in a flame, to break the chemical bond between molecules, freeing individual atoms. In this condition, the atoms can absorb ultraviolet or visible radiation. The wavelength bands which each specific element can absorb are very narrow and are different for every element. The flame used for burning is commonly acetylene with compressed air used as the oxidant. Higher temperatures which are needed for what are known as the "refractory" elements are achieved using nitrous oxide as an oxidant.
If the analyst wishes to determine the concentration of aluminum, for example, he passes the light from a suitable spectral source through the flame. The source is generally a hollow cathode or electrodeless discharge lamp which contains the element of interest, in this case aluminum. A certain portion of the light will be absorbed by free aluminum atoms in the flame, depending on the concentration of aluminum in the sample. The instrument measures the amount of absorption. Modern atomic absorption spectrophotometers can be set to present results directly in concentration values. The analyze extremely small samples, to determine ultra-low concentrations, or to analyze certain solids directly, new sampling devices which augment or replace the flame are available. The most important of these is the graphite furnace or heated graphite atomizer.
As is well known in spectroscopy, the concentration of a radiation absorbing substance is directly proportional to the absorbance A which is defined by the equation A=log [I.sub.0 /I], where I.sub.0 is the intensity of the radiation reaching the sample and I is intensity of the same radiation transmitted by the sample. Typically, the quantities I.sub.0 and I are determined through the use of a double beam type spectrometer in which one beam passes through the sample and the other beam passes around it. Difficulties are encountered, however, in making these measurements and as a result, the relationship between absorbance and concentration deviates from linearity; the deviation is normally referred as curvature and techniques for compensating for this are termed curvature correction, curve straightening or curve compensation.
A discussion of this problem and a solution using analog circuitry is disclosed in U.S. Pat. No. 3,739,164 issued June 12, 1973.
Analog curvature correction systems, although adequate, are difficult to operate requiring numerous settings by the operator or analyst if he is to obtain accurate results. Because each different element exhibits different curvature characteristics and because the curvature changes with changing concentrations, constant adjustment of analog devices is necessary to obtain the desired accuracy of results.
A solution to the problems associated with the use of analog circuitry in such an instrument is proposed in a paper titled "Use of a Small Dedicated Computer in the Design of an Atomic Absorption Spectrophotometer" by T. J. Poulos which was presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 6, 1974. In that article, the author points out the manner in which a microprocessor along with appropriate read-only memories [ROMS] and random access memories [RAMS] can be used to construct a microcomputer which can be used to carry out the data processing in an instrument of this nature including the curvature correction. Briefly, the system discloses integrating the detector signal in an analog integrator, sampling and holding the integrated output, converting it in an analog-to-digital converter and providing the output of the converter as the microcomputer data input. The microcomputer then takes the data input and calculates concentration from the equation: EQU C=k.sub.C1 log 1C2k.sub.C2 [1t.sup.t-k C2]1.sup.t
and absorbance as A=log 1.sup.t where t is the transmittance.
The use of the microcomputer in the instrument offers many advantages as to simplicity and accuracy. However, tests run using the computation scheme proposed in the paper by Poulos using actual measurement data were found to exhibit inaccuracies which are intolerable in some applications. The concentration range covered for good accuracy was found to be small. The use of the scheme in analyzing for cobalt, which exhibits high curvature, produces good results but the results in the determination of copper, which exhibits a fair degree of linearity, were not quite so good. In attempts to handle sodium emission data, there was no success at all. Thus, although the applicability and advantages of using a small dedicated microcomputer in an atomic absorption spectrophotometer has been recognized, instruments developed to date do not exhibit the desired accuracies over the full range of concentrations which must be measured nor are they capable of handling all samples which must be analyzed. In view of this, the need for an improved curvature correction for spectrophotometers which permits operation over wide concentration ranges on a maximmum number of elements, becomes evident.